Ventricular fibrillation is an uncoordinated contraction and relaxation of the individual fibers of the heart which produces no blood flow and results in death unless corrective measures are applied within minutes of onset. Recovery from ventricular fibrillation can be accomplished using drugs or electric shocks. The latter is preferred, mainly because administration of drugs will be of little use in the absence of circulation.
The use of electric shocks to terminate ventricular fibrillation entails passing electric current through the myocardium so as to restore the heart to its natural sinus rhythm. One commonly used method of electric shock therapy involves passing a single burst of electric current through the heart of a patient requiring defibrillation. This single burst may be applied to the heart either transthoracicly from electrodes placed outside the body, or internally from electrodes inside the body normally positioned on, in, or near the heart. Furthermore, the burst of electric current may consist of a monophasic waveform or a multiphasic waveform, for example a biphasic or triphasic waveform. Also, the burst may be applied to one or multiple pathways through the heart depending upon the number of electrodes used and which pulses within the burst are applied across individual electrode pairs. Although efficient in treating the dysrhythmia, the "single burst" method of therapy requires delivering to the patient's heart an electrical pulse of sufficiently high voltage, current and energy that undesirable side effects, such as heart tissue damage and patient discomfort, may result.
To minimize these undesirable side effects, a second method of electric shock therapy utilizes "multiple bursts" of electric current, separated by a fixed time interval. These multiple bursts have consisted of single pulses of electric current separated by a fixed time interval on the order of from 70 to 130 milliseconds. Such "multiple burst" therapy differs from the "single burst" therapy discussed above, in that less voltage, current and energy need be delivered to the patient's heart at any one point in time in order to achieve defibrillation. Accordingly, ventricular defibrillation may be obtained with less patient discomfort and heart tissue damage.
Fibrillation results when many depolarization wavefronts (the locations in the heart where the cell tissue is undergoing depolarization) are moving through the heart in a complicated arrhythmia. When the heart is fibrillating, these depolarization wavefronts pass over any particular portion of the myocardium with a very consistent average timing. The average time interval between successive depolarizations at a particular site in the myocardium is called the fibrillation cycle length.
Prior "multiple burst" defibrillation methods required that the time interval between bursts be some arbitrarily chosen interval. Such methods fail to realize that defibrillation using multiple bursts is optimized when the time interval between bursts is determined by the fibrillation cycle length, rather than by some arbitrary set time interval. Adjusting the time interval between bursts according to the fibrillation cycle length is important since the fibrillation cycle length can vary from mammalian species to mammalian species, from individual to individual within a species, from fibrillation event to fibrillation event in the same individual, and from time to time within the same event.
An object of the present invention is to provide a method of defibrillating the heart of a mammal in need of defibrillation which uses multiple bursts of electrical current delivered to the mammal's heart in a timed relationship to each other which is based upon the mammal's fibrillation cycle length. The instant defibrillation method allows for defibrillation at lower peak voltages and currents, thereby providing defibrillation with a minimum of patient discomfort and heart tissue damage. The instant method's decrease in peak voltages and currents required to accomplish defibrillation also allows improved defibrillator hardware designs and longer defibrillator implant lifetimes.